Texas Instruments LP8556TM-Evaluation module LP8556TM-EVM/NOPB LP8556TM-EVM/NOPB Datenbogen
Produktcode
LP8556TM-EVM/NOPB
7.3. Boost Converter Configuration
The LP8556 boost converter configuration is determined by the size of the LED array to be driven.
Additionally, component constraints will affect operating frequency selection. A calculation tool is available
for selecting the boost operating point and external components along with the desired frequency of
operation.
Additionally, component constraints will affect operating frequency selection. A calculation tool is available
for selecting the boost operating point and external components along with the desired frequency of
operation.
7.3.1. Setting Boost Switching Frequency
Using Resistor Config: The desired frequency of operation is programmed by an external resistor placed
on the FSET pin. The BOOST_FSET_EN bit must be set in the CFG2 register. Note the PWM
_FSET_EN bit must also be set as the PWM output frequency is also set by the resistor on the FSET pin.
Note the BL_ON bit must be set to insure the backlight comes on during operation. See the datasheet
table titled Setting Switching and PWM Dimming Frequency with an External Resistor.
on the FSET pin. The BOOST_FSET_EN bit must be set in the CFG2 register. Note the PWM
_FSET_EN bit must also be set as the PWM output frequency is also set by the resistor on the FSET pin.
Note the BL_ON bit must be set to insure the backlight comes on during operation. See the datasheet
table titled Setting Switching and PWM Dimming Frequency with an External Resistor.
Using I2C Control: The desired frequency of operation is programmed in the BOOST_FREQ bits of the
CFG6 register. The bits BOOST_FSET_EN and PWM_FSET_EN must be reset to zero in CFG2.
CFG6 register. The bits BOOST_FSET_EN and PWM_FSET_EN must be reset to zero in CFG2.
7.3.2. Setting Boost Voltage
The boost voltage can be adaptively set by the LP8556 to control the LED headroom in real time which
provides the most efficient operation. This mode of operation is suggested. The ADAPTIVE bit must be set
in CFG10. When using adaptive mode, the value set in VBOOST of CFG6 is the initial boost output
voltage. VBOOST_MAX must also be set in CFG9 to select the maximum boost voltage allowed during
adaptive mode.
provides the most efficient operation. This mode of operation is suggested. The ADAPTIVE bit must be set
in CFG10. When using adaptive mode, the value set in VBOOST of CFG6 is the initial boost output
voltage. VBOOST_MAX must also be set in CFG9 to select the maximum boost voltage allowed during
adaptive mode.
The boost voltage can be forced by resetting the ADAPTIVE bit to zero. The value in VBOOST of CFG6 is
the boost output voltage. The value in VBOOST_MAX of CFG9 will limit the maximum value that can be
set. This usage is not recommended.
the boost output voltage. The value in VBOOST_MAX of CFG9 will limit the maximum value that can be
set. This usage is not recommended.
7.3.3. Setting Inductor Current Limit
The boost inductor current limit is determined by the inductor used and the amount of power output
required. The limit is set in the IBOOST_LIM bits of the CFG7 register.
required. The limit is set in the IBOOST_LIM bits of the CFG7 register.
7.3.4. EMI Reduction Settings
The boost driver strength is controlled by the EN_DRV2 and EN_DRV3 bits in the CFG7 register. Enabling
all three drivers provides the best efficiency and fastest transition times. It is recommended to have these
bits set to one. Each one could be cleared to reduce the boost output slew rate.
all three drivers provides the best efficiency and fastest transition times. It is recommended to have these
bits set to one. Each one could be cleared to reduce the boost output slew rate.
A spread spectrum edge control feature is also available. This feature changes the switching edge
placement to reduce harmonics of the boost switching frequency. Spread spectrum is enabled by the
SSCLK_EN bit in the CFGA register.
placement to reduce harmonics of the boost switching frequency. Spread spectrum is enabled by the
SSCLK_EN bit in the CFGA register.